1. Field of the Invention
This invention relates generally to the field of leak detection, wherein a leaking fluid causes an impedance change in a sensing cable and the impedance change is detected as an indication of the leak.
2. Related Art
A variety of liquid organic chemicals (e.g., oils, crude oil, refined petroleum products, gasoline, kerosene, organic solvents, and the like) are stored in underground tanks and transported through buried pipelines. Leakage from these tanks and pipelines can contaminate ground water and cause extensive environmental damage. Further, leaks are difficult to detect and often are not detected until extensive environmental damage has already occurred.
One method of detection has been to run a cable adjacent to the underground pipeline or tank. The cable includes a pair of conductors (e.g., coaxial or twin lead) surrounded by a porous insulation such as expanded, microporous polytetrafluoroethylene (ePTFE). ePTFE is hydrophobic (not permeable to liquid water) but permeable to liquid organic chemicals. If a leak occurs, the liquid organic chemical will permeate the insulation and will cause a change in its dielectric properties such that the electrical characteristics (i.e., impedance) between the conductors is changed. This resulting impedance change can be sensed at a remote end of the cable.
U.S. Pat. Nos. 3,981,181 to Ochiai, 4,029,889 to Mizuochi, 4,206,632 to Suzuki, 4,594,638 to Suzuki et al., and 4,877,923 to Sahakian provide examples of conventional leak detection cables.
There are generally two types of monitoring schemes used with leak detection cables: time domain reflectometry (TDR) and capacitance measurement. Both methods are well known in the art. In using the TDR method, one or more electrical pulses are transmitted down a leak detection cable and the cable is monitored for reflections (i.e., return energy from the incident pulses). Reflections are caused by changes in the characteristic impedance of the cable. TDR can be used to precisely determine the occurrence and location of the impedance change (i.e., liquid leak) along the cable.
A decrease in the impedance of a cable resulting from a leak corresponds to an increase in the capacitance of the cable. The change in capacitance can be measured from a remote end of the cable. Capacitance measurement involves charging the leak detection cable with a charging current, monitoring the voltage across the cable and the charge time, and then determining the total capacitance of the cable therefrom. Capacitance measuring circuits tend to be simpler than the TDR circuits for leak detection. However, unlike TDR, the capacitance measuring method is unable to determine the location of the leak.
While conventional leak detection cables are acceptable for most uses, they are often unacceptable for use in a highly humid environment or when submerged in liquid water at elevated temperatures. Under these conditions, the hydrophobic insulation layer(s) may be permeated by water vapors. Materials such as ePTFE have a unique hydrophobic property that allow a similar surface energy liquid (like most hydrocarbon liquids) to permeate them while not allowing permeation by relatively higher surface energy liquids such as water. However, the open pore structure of the materials will allow vapors to enter. These vapors can condense and change the dielectric constant of the insulation layer, resulting in a decrease in the impedance of the leak detection cable. With respect to the capacitance measuring method, this change in impedance will result in an increase in the overall capacitance of the leak detection cable and produce false alarms (i.e., misleading leak detection information).
Even small amounts of water vapor can have a significant impact on the overall dielectric constant of the insulation layer because the dielectric constant of water is, for example, approximately sixty times greater than that of ePTFE.
The capacitance measuring method of leak detection appears to be most susceptible to false alarms resulting from water vapor penetration.
What is needed is a leak detection cable for detecting liquid organic chemicals which is not susceptible to permeation by water vapors.